Recently, an optical information storage medium processes a vast amount of information such as images, and therefore further increase of information recording density has been demanded. In order to satisfy the demand, the following techniques have been suggested: a super resolution technique and a multi-layer optical information storage medium. The super resolution technique is one of the techniques for improving information reproduction processing, and the multi-layer optical information storage medium is a medium having multiple information storage layers, each of which is recordable and reproducible individually. The super resolution technique is a technique for reproducing a signal shorter in mark length (the mark length depends upon an optical system and numerical apertures of a laser wavelength) than a resolution limit of an optical system of the reproducing apparatus. With this technique, information recording using a shorter mark length becomes possible, and therefore substantial recording density increases. This is due to the fact that a reproduction technique, not a recording technique, is an issue for increasing the recording density.
First, the super resolution technique will be described below. Conventionally, various optical information storage media (hereinafter, the media will be referred to as super resolution media) for reproducing a signal shorter in mark length than a resolution limit of an optical system of a reproducing apparatus have been suggested.
For example, a Japanese unexamined patent publication (Document 1) teaches a technique which enables an optical storage medium driving apparatus to reproduce a rewritable optical magnetic storage medium in which information is stored in a magnetic storage film in a magnetization direction. This technique is not applicable to reproduction of a read-only medium in which non-rewritable information is stored in the form of projections/depressions on a substrate. Another well-known technique may be a medium using a thermochromic dye mask layer whose optical characteristic (transmittance) changes depending upon temperature, which is provided on a reproduction light incident side of a reflection film (Document 2). This technique is applicable both to a rewritable optical storage medium and a read-only optical storage medium. The mask layer designates a layer inducing super resolution phenomenon, such as the following artificial reduction of laser spot.
The optical information storage medium used in the foregoing technique utilizes the fact that a laser spot produced by a reproduction laser emitted on a reproduction face generates a light intensity distribution, which results in temperature distribution. More specifically, in the optical magnetic storage medium according to Document 1, a reproduction layer is disposed on a storage layer, and a magnetic field generated in the storage layer in the reproduction process is transcribed only to a certain portion of the reproduction layer that corresponds to a high temperature portion of the laser spot. This makes it possible to reproduce a signal shorter in mark length than the resolution limit of the optical system. Further, in the optical storage medium according to Document 2, the temperature distribution or the light intensity distribution is generated within a reproduction laser spot produced on a reproduction layer that is closer to a reproduction light incident side than a reflection layer. As a result, the laser spot has optical characteristic distribution. For example in a case where the reproduction layer is made of material whose transmittance increases as temperature increases, only the transmittance of the high temperature portion increases. This artificially reduces the laser spot produced on the reflection layer, making it possible to reproduce the signal shorter in mark length than the resolution limit of the optical system.
The following describes the multi-layer optical information storage medium. In the multi-layer optical information storage medium, for example as disclosed in Document 3, a plurality of information storage layers are layered from a reproduction light incident side, in such a way as to dispose a first information storage layer from the reproduction light incident side, a second information storage layer thereon, and so on, therebetween having an intermediate layer mainly made of resin. In this structure, the information storage layers, except for the farthest information storage layer from the reproduction light incident side, are translucent layers, thereby allowing the reproduction light to be transmitted. Therefore, the reproduction light incident from the reproduction light incident side is focused on each of the information storage layers. Accordingly, the information recording density of the multi-layer optical information storage medium can be increased by increasing the number of the information storage layers.
As described above, these two methods have been suggested for increasing recording density of optical information storage media.
Document 1: Japanese Unexamined Patent Publication No. 180486/1996 (Tokukaihei 8-180486, published on Jul. 12, 1996)
Document 2: Japanese Unexamined Patent Publication No. 35012/2001 (published on Feb. 9, 2001)
Document 3: Japanese Unexamined Patent Publication No. 235733/2000 (published on Aug. 29, 2000)
However, in the super resolution reproduction technique, the laser spot is artificially reduced, and therefore the utilization of the reproduction light becomes less efficient (the reflection light is certainly reduced). This limits the reduction of the laser spot, and therefore significant increase of the recording density cannot be expected (maximally to a double in line density).
Moreover, in many optical information storage media employing the super resolution reproduction technique, such as that disclosed in Document 2, that is applicable to a read-only optical information storage medium, the mask layer used therein causes a composition change or a phase change by directly absorbing light or heat. This tends to heavily burden the mask layer material, weakening the reproduction durability.
Further, normally, a reproduction film used in an optical information storage medium that adopts the super resolution reproduction technique to reproduce a signal shorter in mark length than the resolution limit of the optical system of the reproducing apparatus (hereinafter, the optical information storage medium will be referred to as a super resolution medium) is made of dye or phase change material. The dye or the phase change material is more expensive than film material normally used in an optical information storage medium. This causes a problem that the super resolution medium becomes more expensive than an ordinary optical information storage medium (with a single-layer information storage layer).
Meanwhile, the multi-layer optical information storage medium requires complex manufacturing processes, which extremely increases the cost. The following describes reasons therefor, with reference to an exemplary production method of the multi-layer optical information storage medium.
Production of the multi-layer optical information storage medium begins with formation of a first information storage layer, such as a storage film or a reflection film, on a substrate usually by, for example, sputtering in vacuum. Then back in the atmosphere, the first information storage layer is coated with, for example, UV-curable resin by spin coating. Then, a plastic stamper is bonded thereto, which is removed after curing the resin by ultra violet irradiation. As a result, grooves for tracking or irregularity such as pre-pits that represent information by its configuration are transcribed onto the surface of the intermediate layer (so-called a 2P process). Further, the second information storage layer is formed on the intermediate layer, and another intermediate layer on which irregular pattern is transcribed by the 2P process is layered thereon. The above steps are repeated according to the desired number of the information storage layers. At the end, a cover layer (light transmitting layer) is formed thereon.
As described above, the multi-layer optical information storage medium is manufactured through extremely complex steps in which the multi-layer optical information storage medium is passed on between the vacuum and the atmosphere many times. Moreover, the information storage layers have different film structures so as to adjust their reflectances. Therefore, in ordinary mass production in which the various layers are formed by transferring the medium in one way of production line, the vacuum film-forming apparatuses are needed as many as the number of the information storage layers. Moreover, a vacuum film-forming apparatus is extremely expensive, and its running costs are expensive relative to the other apparatuses used for producing optical information storage media.
Because of the above reasons, the multi-layer optical information storage medium becomes extremely expensive. This is apparent from the current price (Jan. 31, 2005) of a single-sided two-layer Blu-ray Disc (storage capacity: 50 GB) that is more than a double of that of a single-layer Blu-ray Disc (storage capacity: 25 GB). Usually, complication of production steps increases the costs much more significantly than a change of material of the information storage layers does.
As described above, the conventional methods for increasing information recording density in the optical information storage medium have many problems.